docs: add hint for custom root types

[mikuhn]

Change Summary

Updated docs regarding the access of __root__ with custom root types.

Related issue number

#1372

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1373 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1373   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3723      3723           
  Branches       735       735           
=========================================
  Hits          3723      3723           

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[samuelcolvin]

thanks so much.